US3355906A - Refrigeration system including control for varying compressor speed - Google Patents

Description

A. B. NEWTON REFRIGERATION SYSTEM INCLUDING CONTROL Dec. 5, 1967 FOR VARYING COMPRESSOR SPEED Filed Nov. 8, 1965 United States Patent l 3,355,906 REFRIGERATION SYSTEM INCLUDING CONTROL FOR VARYING COMPRESSOR SPEED Alwin B. Newton, York, Pa., assignor to Borg-Warner Corporation, Chicago, 111., a corporation of Illinois Filed Nov. 8, 1965, Ser. No. 506,661 8 Claims. (Cl. 62-209) ABSTRACT OF THE DISCLOSURE A refrigeration system including a compressor speed control adapted to drive the compressor at a speed which will result in maximum operating efiiciency. The control system includes appropriate pressure or temperature sensing elements and a circuit deriving a signal which is a function of the ratio between discharge and suction pressure in the system. This signal is then applied to the control circuit which in a preferred embodiment includes a solid state motor speed control for an electric motor.

This invention relates generally to vapor cycle refrigeration apparatus and more particularly to a control system operative to vary compressor speed as a function of the ratio between discharge and suction pressure to obtain maximum economy and stability.

In refrigeration systems, particularly those of the type which utilize a centrifugal compressor, it is conventional to vary the capacity of the compressor in response to changes in the cooling load by sensing some condition which is indicative of the magnitude of cooling load imposed on the system, such as the leaving or entering chilled water temperature or evaporator temperature. Capacity control is commonly achieved by a mechanism known as a pre-rotation vane control, hereinafter referred to as PRV, which varies compressor capacity not only by throttling the flow of suction'gas, but also by varying the angle at which the suction gas is directed into the compressor wheel.

The suction and discharge pressure are established for normal, full load operation as afunction of the expected condenser water temperature and chilled water temperature respectively, thus establishing the compression ratio on which the compressor design and operating speed is based. Since suction and discharge pressuresvary both with changes in the load and condenser water temperature, it is unlikely that the compressor is required to operate at these specific design condition over much of the operating period. If the capacity control is achieved by pure suction gas throttling, the suction pressure seen by the impeller is reduced, thus increasing the compression ratio and in turn reducing the efficiency of the compressor. If, however, a PRV unit is used and the position of the vanes in the aforementioned PRV unit is varied, the capacity, at least in the upper control range, can be varied with little, if any, effect on the suction pressure (and hence 5 the compression ratio).

Electric motor driven compressors, and particularly hermatic units, are forced to operate at a constant speed, since induction motors run at nearly synchronous speed,

as determined by frequency of alternating current and number of motor poles. Two problems are, therefore, imposed on such a system: first, the compression ratio imposed on the compressor by existing evaporator and condensingtemperature (and corresponding pressures) is not the :compression ratio at whichthe wheels operate efiiciently, except at one speed; and second, the PRV unit is'sometimes forced to operate beyond its normal travel for changing the angle of gas How and thus becomes a throttling valve..;Since vcompressor performance is very sensitive to both changes in the angle of gas approaching and to gas throttling, the action just described results in unstable and inefficient operation of the system.

From the foregoing, We see that for any given pressure ratio of suction and discharge pressure, there is a unique compressor speed which will achieve maximum economy during operation. Even with many of the best compressor wheel designs, this relationship between speed and compression ratio is extremely critical for stable operation. The present invention is directed to a control system which avoids the shortcomings of known prior art systems in that means are provided for continuously sensing the ratio between discharge and suction pressure and modulating the speed of the compressor in response to the sensed ratio so that optimum economy and stability is achieved for any set of operating conditions.

It is therefore, a principal object of the invention to provide a refrigeration system in which the compressor speed is continuously varied in response to changing requirements for the pumping head which will occur as the load or condensing temperatures vary during operation. Another object of the invention is to provide an improved compressor motor drive system which varies the speed of the compressor in accordance with a signal derived from the pressure ratio imposed on the compressor. Another object of the invention is to provide a compressor with a motor speed control unit which is simple in construction and economical in operation. Additional objects and advantages will be apparent from reading the following detailed description taken in conjunction with the drawings where in the figure is a diagrammatic representation of a preferred embodiment of the invention.

Referrin now to the figure, the basic components of a preferred refrigeration system include a compressor C, a condenser R, and an evaporator E connected to provide a closed refrigeration circuit, a motor M driving the compressor, means F for modulating the speed of the compressor and means D- for deriving a control signal which is a function of the ratio between discharge and suction pressures.

Discharge gas from the compressor is forwarded to the condenser through hot gas line 10; and the condenser and evaporator are connected by line 12 containing a float or expansion valve 14. The suction side of the compressor is connected to the evaporator E by means of a line 16. Although the drawing illustrates a system for cooling a secondary heat exchange medium in the evaporator, a liquid heat exchange medium being circulated to and from a load, such a a plurality of induction or fan coil units, through a tube bundle 18 in the evaporator or chiller B, it should be understood that the invention is not to be restricted to refrigeration systems for liquid chillers.

There areseveral different arrangements for effecting the control of compressor capacity; and one well known system employs a pre-rotation vane (PRV) mechanism including a plurality of vanes 20 placed in the path of suction gas entering the compressor wheel. The vanes are pivotably supported along corresponding radial axes so that their movement selectively controls the angular deflection imparted to the suction gas passing therethrough. Since the capacity of the compressor is dependent on the relative angle between the direction of suction gas flow and the pitch of the compressor vanes, the capacity of the compressor may be effectively varied by such PRV mechanism. A servo element 22, or its equivalent, for changing the position of the vanes may be operated in response to the temperature of the heat exchange medium leaving the evaporator or tube bundle by temperature responsive controller 24. It should be understood that other control systems may be employed; for example, a wide differential control on the return water instead of the leaving water 3 from the evaporator is frequently used, and may be a more stable control. It is also possible to control the PRV by the temperature difference across the supply water and return water from and to the evaporator.

The compressor drive motor, designated generally at M, includes an output shaft 26 coupled in drivin relation to the compressor wheel 28. For simplicity, the motor is shown as being of the single phase type, it being understood that the same control approach may be adapted for the more typical three-phase power systems used in larger motors.

The motor, compressor, and electrical control circuit components shown in the figure include means, generally designated at F, for regulating the speed of motor M in response to a control signal, derived by means D, which is a function of the ratio between the input pressure and output pressure imposed on compressor C. The motor speed control may comprise any one of several known systems. However, in a preferred embodiment, the basic components of such a system include an inverter circuit and a unijunction firing circuit of the type described in General Electric Co. SCR Manual, 2nd Ed. (1961) Fig. 9.7. These inverter and firing circuits are effective in controlling the frequency of the AC output supplied to the field coil windings of the motor and thereby vary the speed of the motor without appreciable power losses.

Means generally designated at D are provided for deriving a control signal which is a function of the ratio between the discharge pressure and the suction pressure of the compressor. This control signal is then transmitted to the invert 'r and unijunction firing circuit to control the speed of e compressor. Conduits 30, 31 are utilized to transmit indications of the suction and discharge pressures for conversion in a ratio circuit to an electrical signal connoting this ratio. The electrical signal in turn regulates the frequency of conduction of unijunction transistor 34, which is operative in its turn to regulate the frequency of operation of the inverter circuit comprising SCRs 36, 38. Since AC motor M is driven at a speed related to the frequency of the AC energy applied thereto over conductors 39, 40, it is apparent that the motor speed is a function of the ratio between the discharge and suction pressures of compressor C.

In more detail, the pressure-to-electrical signal conversion unit D includes a transformer 42 having a core 43, a primary winding 44 with a movable tap or slider arm 45, and a secondary winding 46 with another movable slider arm 47. The positions of arms 45, 47 determine the effective turns ratio of transformer 42.

AC energy is applied to unit D over conductors 48, 49 from a conventional power source (not shown). Conductor 48 is coupled to movable slider arm 45, and conductor 49 is coupled to one end portion of primary winding 44. A bellows 50 is coupled between conduit 30 and slider arm 45 to convert the pressure indication received in line 30 into a mechanical displacement of arm 45 in a well known manner. Accordingly, the effective number of turns of primary winding 44 is a function of the suction pressure P of compressor C as measured in line 16 or another suitable location.

The discharge pressure of the compressor (Pd), as measured in the volute or other suitable location is applied through conduit 31 to another bellows 52, which is operative to position slider arm 47 along secondary winding 46 in a position related to the compressor discharge pressure. In order to assure that the pressure measurements are truly indicative of the absolute pressure in the discharge and suction lines, reference pressure bellows 50, 52' are mechanically linked by rods 56, 57, respectively, to bellows 50 and 52. Bellows 50' and 52 are evacuated to provide a zero reference pressure P so that any movement of the sliders reflects the absolute suction and discharge pressures existing in the compressor, bellows 50, 52 being biased by springs 51, 58.

The output in the secondary winding is converted to DC by having the slider arm 47 coupled over conductor 53 to one input connection of a rectifier bridge 54, comprised of four diodes 55. One end of secondary winding 46 is coupled to the other input connection of bridge 54 via conductor 59, and the output connections of this bridge are coupled to conductors 6t) and 61.

The oscillator or timing circuit including unijunction transistor 34 is coupled between a conductor 62, to which a positive potential is applied, and ground conductor 70. A first series circuit extends from conductor 62 over resistor 64, the B2 to BI circuit to transistor 34, and primary winding 66 of pulse transformer 68, which includes a core 69. The other end of winding 66 is coupled to ground conductor 70. A second series circuit extends from conductor 62 over resistor 71 and capacitor 72 to conductor 70. The emitter of transistor 34 is coupled to the common connection between resistor 71 and capacitor 72.

Power for the inverter circuit, which includes SCRs 36, 38 is provided by a rectifier bridge 74, including four diodes 75. Input conductors L L pass alternating energy from a suitable AC source (not shown) to the bridge, and one of the output conductors is coupled from an output connection of rectifier bridge 74 to the anode 36a of SCR 36, and is also coupled to one plate of capacitor 82. The other side of capacitor 82 is coupled to a common reference point 84. The other output conductor 81 of rectifier bridge 74 is coupled to one side of a temperature responsive switch 86, sensing the temperature of leaving chilled water through control line 87. When the leaving chilled water temperature rises above a predetermined value, switch 86 is closed to initiate operation of the compressor by closing circuit between 81and conductor 70.

The cathode 360 of SCR 36 is coupled to a commutating choke 90 which includes a core 91, an upper portion 92 and a lower portion 93. The lower part of the commutating choke is coupled through the anode-cathode path of SCR 38 to ground conductor 70. A resistor 74 is coupled between reference point 84 and the center tap of choke 90, and a capacitor 96 is coupled between point 84 and ground. The ground connection is also extended over conductor 39 to one terminal on motor M, and the other energizing conductor 40 of this motor is coupled to point 84.

Operation In operation, AC energy is applied over conductors 48, 49, and over conductors L L and an appropriate DC energizing potential is applied between conductors 62, 70. Application of the alternating energy over conductors 48, 49 establishes a reference potential difference between the left end of primary winding 44 and the connection of movable tap 45.

If the compressor is not operating, the suction and discharge pressures will be equal. Consequently, the control signal generating circuit D will be calling for zero capacity. This requires a starting system to initiate operation of the compressor. The secondary winding 46 contains a number of additional turns, 46a, which are put into operation by means of a two-position start-run switch 97. Switch 97 is operated by means of a controller 98 which is actuated in turn by means 100 for sensing motor current, or a parameter related to current, such as motor speed as sensed at 102. This control really serves two functions: first, it provides the necessary frequency over and above that normally required for getting the system started; and secondly, it can be used to relate the starting; frequency and the electrical impedance characteristics of the motor so that this deviation in frequency canbe employed to reduce the starting inrush. Switch 97 is initially place-d in the start position so that the additionl turns 46a are in the circuit. After the compressor is running, the starting period may be terminated by a timer or a speed or motor current sensing device which is operative to shift switch 97 to the run position. The effect of the above arrangement is to increase the relative number of turns so as to provide a relatively high frequency for starting the compressor. The action of this switch could also be terminated at some percentage of full load speed, probably between 20% and 80%.

As compressor C is operated, the suction pressure is passed through conduit 30 and converted by transducer 50 into a linear position of arm 45, which position varies as a function of variations in the suction pressure. Concomitantly, the discharge pressure Pd from the compressor C is applied through conduit 31 and converted in transducer 52 into a mechanical displacement which regulates the position of movable tap 47 on secondary winding 46. Accordingly, the alternating output signal from transformer 42, which appears between the left end of secondary winding 46 and movable tap 47, denotes the ratio of discharge pressure to suction pressure of compressor C. This electrical signal, indicative of the pressure ratio, is applied across the input connections of rectifier bridge 54 which operates in a well known manner to produce a related unidirectional signal between output conductors 60, 61. Considering the frequency-determining circuit including unijunction transistor 34, normally the rate at which transistor 34 is fired is determined by capacitor 72, resistor 71, and by resistor 64. As capacitor 72 charges through resistor 71, the voltage applied to the emitter of transistor 34 gradually goes more positive until this transistor fires. However, the voltage at the emitter is modified by the connection of the output terminals of rectifier bridge 54 across capacitor 72. Accordingly, the frequency-determining circuit is operative to produce a timing signal related to the ratio of pressures described previously.

Each time unijunction transistor 34 fires, current is passed through a circuit including primary winding 66 of pulse transformer 68; and corresponding pulses of current are provided in secondary windings 67a, 67b of the pulse transformer. These pulses are applied to the gates 36g, 38g of the SCRs 36 and 38 respectively. The application of these gating pulses, together with the commutating choke 90, functions in a Well known manner to elfect the alternate conduction of SCRs 36 and 38 to sequentially reverse the direction of the energy passed over conductors 3'9, 40 to motor M. D.C. energy is supplied to the SCRs by operation of rectifier bridge 74 as soon as the system is energized. Accordingly, it is manifest that the speed of operation of motor M is a function of the ratio of the discharge pressure to the suction pressure of compressor C.

While this invention has been described in connection with a certain specific embodiment thereof, it is to be understood that this is by way of illustration and not by way of limitation; and the scope of this invention is defined solely by the appended claims which should be construed as broadly as the prior art will permit.

What is claimed is:

1. A refrigeration system comprising a compressor, a condenser, and an evaporator connected to provide a closed refrigeration circuit; infinitely variable drive means op'erativelyconnected to drive said compressor; means for deriving a control signal which is a. function of the ratio between the discharge pressure and the suction pressure of said compressor; and regulating means for modulating the speed of said infinitely variable drive means in response to said control signal.

2.. Apparatus as defined in claim 1 wherein said infinitely variable drive means comprises an electrical motor and 'said regulating means includes means for varying the frequency of the electrical energy supplied to said motor.

3. Apparatus as defined in claim 2 including means for increasing the frequency of the electrical energy supplied to said motor upon initial energization of said motor, and means for subsequently rendering said last named means inoperative and transferring control of said regulating means back to said control signal.

4. A refrigeration system comprising a compressor, a condenser, and an evaporator connected to provide a closed refrigeration circuit, an AC electrical motor driving said compressor, a power supply for said motor including an inverter circuit and a firing circuit, a pressure to electrical signal transducer for deriving a control signal which is a function of the ratio between discharge pressure and suction pressure of said compressor, and means coupling said control signal to said firing circuit to control the frequency of the electrical energy supplied to said motor in response to said control signal, whereby the compressor is driven at a predetermined speed for any given pressure ratio.

5. Apparatus as defined in claim 4 wherein means are provided for increasing the frequency of the electrical energy supplied to said motor during initial start-up thereof, and means for rendering said last named means inoperative in response to a predetermined signal.

6. Apparatus as defined in claim 5 wherein said predetermined signal is derived from the flow of electrical energy to said motor.

7. Apparatus as defined in claim 5 wherein said predetermined signal is derived from the speed of said compressor.

8. A refrigeration system comprising: a compressor, a condenser, and an evaporator connected to provide a closed circuit refrigeration system; means for varying the capacity of said compressor; means for deriving an electrical signal which is a function of the ratio between the discharge pressure and the suction pressure of said compressor; means for actuating the capacity varying means in response to the cooling requirements of said refrigeration system and means for independently controlling the speed at which said compressor is driven in response to said electrical signal.

References Cited UNITED STATES PATENTS 1,750,336 3/1930 Terry 62---209 1,943,318 1/1934 Hulse 62209 2,290,426 7/ 1942 Haines 62----209 MEYER PERLIN, Primary Examiner.

Claims (1)

1. A REFRIGERATION SYSTEM COMPRISING A COMPRESSOR, A CONDENSER, AND AN EVAPORATOR CONNECTED TO PROVIDE A CLOSED REFRIGERATION CIRCUIT; INFINITELY VARIABLE DRIVE MEANS OPERATIVELY CONNECTED TO DRIVE SAID COMPRESSOR; MEANS FOR DERIVING A CONTROL SIGNAL WHICH IS A FUNCTION OF THE RATIO BETWEEN THE DISCHARGE PRESSURE AND THE SUCTION PRESSURE OF SAID COMPRESSOR; AND REGULATING MEANS FOR MODULATING THE SPEED OF SAID INFINITELY VARIABLE DRIVE MEANS IN RESPONSE TO SAID CONTROL SIGNAL.

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